Recently, polyarylene sulfides have been noted as plastics having melt molding properties and heat resistance, and have been widely utilized in various fields. For example, they are utilized in the production of various molding articles by subjecting them to injection molding or extrusion molding, and further, fibers and films are produced by subjecting them to melt extrusion and then, optionally, orienting and heat setting the extruded products. Furthermore, they are utilized in protective films of metals such as sealing compounds or coatings of IC and transistors by subjecting them to injection molding.
As a process for preparing polyarylene sulfides (hereinafter referred to as PPS), Japanese Patent Publication No. 3368/1970 discloses a reaction of an alkali sulfide and a dihalo-substituted aromatic compound in an organic polar solvent such as N-methylpyrrolidone. A polymer prepared by such a process has low melt viscosity and, therefore, it is difficult to extrude by melt extrusion. Even if it is extruded, further difficulty is encountered in the production of fiber or film having high strength by orienting it after melt extrusion. Accordingly, a procedure for increasing melt viscosity has been employed, wherein a somewhat larger amount of the polyhalo-substituted aromatic compound is added to the polymerization reaction, or heating is effected after the polymerization reaction to proceed branching and cross linking reaction.
On the other hand, in order to increase melt viscosity or intrinsic viscosity without employing the above procedure, the use of various polymerization catalysts has been investigated. For example, the use of alkali metal carboxylates is proposed in Japanese Patent Publication No. 12240/1977 and the use of fatty acid alkaline earth metal salts is proposed in Japanese Patent Laid Open Publication No. 40738/1980. U.S. Patent No. 4,038,260 discloses alkali metal salts of organic sulfonic acids: Japanese Patent Laid Open Publication No. 43139/1980 discloses alkali metal salts of specific aromatic sulfonic acids: Japanese Patent Laid Open Publication No. 20030/1981 discloses tri-alkali metal salts of phosphoric acid: and Japanese Patent Laid Open Publication No. 20031/1981 discloses di-alkali metal salts of phosphonic acid. Further, Japanese Patent Laid Open Publication No. 22113/1983 discloses metal salts of organic carboxylic acids or organic sulfonic acids: and Japanese Patent Laid Open Publication No. 206632/1983 discloses combination of organic carboxylic acids or sulfonic acids and alkali hydroxides. In addition, Japanese Patent Laid Open Publication No. 25822/1984 discloses the use of alkali metal salts of organic carboxylic acids together with a polyoxyethylene ether to reduce a content of electrolytes.
By using these catalysts, it is possible to increase melt viscosity or intrinsic viscosity, or to reduce a content of electrolytes.
However, in order to obtain PPS having high melt viscosity by using these catalysts, it is required to use the catalysts in a relatively larger amount such as about 0.2 to 1 mole based on the alkali sulfide. Further, when these catalysts are used, it is difficult to reduce a metal content in a polymer by employing a conventional washing with hot water, acetone and the like.
Usually, PPS bubbles slightly or, depending upon conditions, vigorously during melt extrusion and injection steps. This is caused by low heat resistance of the polymer and, generally, it is considered that this is resulted from oxidation of sulfur atom in the polymer to generate sulfur dioxide. In order to prevent this, addition of various additives has been also proposed.
Further, generally, PPS is obtained as a slightly colored material and, according to a particular polymerization catalyst used, it is colored in pale brown to pale off-white. Heretofore, it has not been succeeded in the production of PPS in pure white because coloring itself occurs during polymerization and it is difficult to increase whiteness after the polymerization, while heat resistance can be improved, and progress of coloring in the later stages can be controlled by addition of an additive. When whiteness of a polymer is inferior, fibers and films as well as molded articles produced therefrom can hardly be marketed and, therefore, it has been requested to produce a polymer which has great whiteness after the polymerization reaction.
As is seen from the above, in a conventional process, it has been very difficult to obtain PPS which meets all the requirements such as high melt viscosity or intrinsic viscosity, a low content of electrolytes, great whiteness and high heat stability, and can be polymerized stably and economically by using a smaller amount of a polymerization catalyst.
Under these circumstances, the present inventors have intensively studied to obtain PPS which can meets these requirements. As the result, it has been found that desired PPS can be obtained by using certain polymerization catalysts.